Exhaust emission control device and method for internal combustion engine, and engine control unit

Abstract

An exhaust emission control device which is capable of supplying a just enough amount of reducing agent to a NOx catalyst while causing the amount of reducing agent consumed in an upstream catalyst to be reflected thereon. A NOx catalyst for trapping NOx and reducing NOx to change the same into harmless ingredients in a reducing atmosphere is disposed downstream of a catalyst. Reducing agent is supplied upstream of the catalyst to cause the NOx catalyst to perform a NOx-reducing action. The amount of oxygen occluded in the catalyst is calculated. During reduction control, according to the oxygen occlusion amount, the amount of reducing agent oxidized and consumed in the catalyst is calculated. According to the reducing agent consumption amount, the amount of reducing agent supplied to the NOx catalyst is calculated. The reduction control is terminated based on the reducing agent supply amount.

1. An exhaust emission control device for an internal combustion engine, for purifying exhaust gases discharged from the engine into an exhaust system, comprising:
 a catalyst that is disposed in the exhaust system and has an oxygen occlusion function and an oxidation function for purifying exhaust gases; 
 a NOx catalyst that is disposed in the exhaust system at a location downstream of the catalyst, for trapping NOx contained in exhaust gases in an oxidizing atmosphere, and reducing the trapped NOx in a reducing atmosphere to thereby purify the trapped NOx; 
 NOx reduction control means for executing reduction control in which the exhaust gases flowing into said NOx catalyst are controlled to the reducing atmosphere, by supplying reducing agent to an upstream side of said catalyst so as to cause said NOx catalyst to carry out an operation for reducing NOx; 
 oxygen occlusion amount-calculating means for calculating an amount of oxygen occluded in said catalyst as an oxygen occlusion amount; 
 reducing agent consumption amount-calculating means for calculating an amount of reducing agent consumed by oxidation in said catalyst during execution of the reduction control by the NOx reduction control by said NOx reduction control means, as a reducing agent consumption amount, according to the calculated oxygen occlusion amount; 
 reducing agent supply amount-calculating means for calculating an amount of reducing agent supplied to said NOx catalyst, as a reducing agent supply amount, according to the calculated reducing agent consumption amount; and 
 reduction control-terminating means for terminating the reduction control when the calculated reducing agent supply amount exceeds a threshold value. 
 
     
     
       2. An exhaust emission control device as claimed in  claim 1 , further comprising catalyst temperature-detecting means for detecting a temperature of said catalyst, and
 wherein said oxygen occlusion amount-calculating means calculates the oxygen occlusion amount according to the detected temperature of the catalyst. 
 
     
     
       3. An exhaust emission control device as claimed in  claim 1 , further comprising:
 upstream air-fuel ratio-detecting means for detecting an air-fuel ratio upstream of said catalyst; and 
 downstream air-fuel ratio-detecting means for detecting an air-fuel ratio downstream of said catalyst, and 
 wherein said oxygen occlusion amount-calculating means calculates the oxygen occlusion amount according to a difference between the detected upstream air-fuel ratio and the detected downstream air-fuel ratio. 
 
     
     
       4. An exhaust emission control method for purifying exhaust gases discharged from an internal combustion engine into an exhaust system, the engine including a catalyst that is disposed in the exhaust system and has an oxygen occlusion function and an oxidation function for purifying exhaust gases, and a NOx catalyst that is disposed in the exhaust system at a location downstream of the catalyst, for trapping NOx contained in exhaust gases in an oxidizing atmosphere, and reducing the trapped NOx in a reducing atmosphere to thereby purify the trapped NOx, the exhaust emission control method comprising:
 a NOx reduction control step of executing reduction control in which the exhaust gases flowing into the NOx catalyst are controlled to the reducing atmosphere, by supplying reducing agent to an upstream side of the catalyst so as to cause the NOx catalyst to carry out an operation for reducing NOx; 
 an oxygen occlusion amount-calculating step of calculating an amount of oxygen occluded in the catalyst as an oxygen occlusion amount; 
 a reducing agent consumption amount-calculating step of calculating an amount of reducing agent consumed by oxidation in the catalyst during execution of the reduction control by the NOx reduction control in said NOx reduction control step, as a reducing agent consumption amount, according to the calculated oxygen occlusion amount; 
 a reducing agent supply amount-calculating step of calculating an amount of reducing agent supplied to the NOx catalyst, as a reducing agent supply amount, according to the calculated reducing agent consumption amount; and 
 a reduction control-terminating step of terminating the reduction control when the calculated reducing agent supply amount exceeds a threshold value. 
 
     
     
       5. An exhaust emission control method as claimed in  claim 4 , further comprising a catalyst temperature-detecting step of detecting a temperature of the catalyst, and
 wherein said oxygen occlusion amount-calculating step includes calculating the oxygen occlusion amount according to the detected temperature of the catalyst. 
 
     
     
       6. An exhaust emission control method as claimed in  claim 4 , further comprising:
 an upstream air-fuel ratio-detecting step of detecting an air-fuel ratio upstream of the catalyst; and 
 a downstream air-fuel ratio-detecting step of detecting an air-fuel ratio downstream of the catalyst, and 
 wherein said oxygen occlusion amount-calculating step includes calculating the oxygen occlusion amount according to a difference between the detected upstream air-fuel ratio and the detected downstream air-fuel ratio. 
 
     
     
       7. An engine control unit including a control program for causing a computer to execute an exhaust emission control method for purifying exhaust gases discharged from an internal combustion engine into an exhaust system, the engine including a catalyst that is disposed in the exhaust system and has an oxygen occlusion function and an oxidation function for purifying exhaust gases, and a NOx catalyst that is disposed in the exhaust system at a location downstream of the catalyst, for trapping NOx contained in exhaust gases in an oxidizing atmosphere, and reducing the trapped NOx in a reducing atmosphere to thereby purify the trapped NOx,
 wherein the control program causes the computer to execute reduction control in which the exhaust gases flowing into the NOx catalyst are controlled to the reducing atmosphere, by supplying reducing agent to an upstream side of the catalyst so as to cause the NOx catalyst to carry out an operation for reducing NOx, calculate an amount of oxygen occluded in the catalyst as an oxygen occlusion amount, calculate an amount of reducing agent consumed by oxidation in the catalyst during execution of the reduction control by the NOx reduction control, as a reducing agent consumption amount, according to the calculated oxygen occlusion amount, calculate an amount of reducing agent supplied to the NOx catalyst, as a reducing agent supply amount, according to the calculated reducing agent consumption amount, and terminate the reduction control when the calculated reducing agent supply amount exceeds a threshold value. 
 
     
     
       8. An engine control unit as claimed in  claim 7 , wherein the control program causes the computer to detect a temperature of the catalyst, and calculate the oxygen occlusion amount according to the detected temperature of the catalyst. 
     
     
       9. An engine control unit as claimed in  claim 7 , wherein the control program causes the computer to detect an air-fuel ratio upstream of the catalyst, detect an air-fuel ratio downstream of the catalyst, and calculate the oxygen occlusion amount according to a difference between the detected upstream air-fuel ratio and the detected downstream air-fuel ratio.